FM radio is in wide use in the field of radio broadcast. The term FM includes, for example, any of the Frequency Modulation methodologies used or developed for signal broadcasting in a frequency band assigned by the U.S. Federal Communications Commission (FCC), nominally in the transmission range 88 MHz to 108 MHz, which is near the middle of the Very-High-Frequency (VHF) television broadcast band. These Frequency Modulation technologies include both analog FM and digital FM.
The FCC has adopted a standard for analog-digital FM transmission called the iBiquity IBOC (In-Band-On-Channel) for hybrid analog-digital transmission systems. According to the IBOC standard, FM stations in the United States must be able to simultaneously broadcast analog and digital signals within their current allocated frequency range. One approach for achieving the above simulcast is to use two separate transmission systems (i.e., analog-digital) to feed two separate antennas (i.e., analog-digital). Since the elevation of the antenna on the tower directly affects the antenna's coverage, it would be desirable to co-locate the analog and digital signals at the same height above the ground to maintain the same coverage.
Also, since the azimuthal pattern of an FM antenna is very dependent on the cross section of the tower structure, it would be desirable to mount both the analog and digital antennas in the same orientation with respect to the tower. When adding digital coverage, concerns are that many towers are already full having no additional aperture space available. Therefore, many FM broadcasters have responded by vertically interleaving the second digital antenna within the aperture of their existing analog antenna. One challenge to overcome when antennas are placed in this configuration is a practical feed system which allows for both systems to occupy the same aperture space without deleteriously altering the characteristics of the pre-existing antenna system.
Accordingly, it is desirable to provide systems and methods which enable an antenna system added to the same general aperture space of a pre-existing antenna system to be fed without extensive design modifications or interference with the pre-existing antenna system.